1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, a printer, a facsimile machine, and a complex machine having the functions of these apparatuses, and a fixing device provided in the image forming apparatus.
2. Discussion of the Background Arts
In general, electrophotographic image forming apparatuses, such as copiers, printers, facsimile machines, or multifunction devices including at least two of those functions, include an image forming unit to form an image on an image carrier, a transfer unit to transfer the image from the image carrier onto a sheet of recording media, and a fixing device to fix the image on the sheet.
The fixing device is a mechanism that typically includes a pressure member, a heating member such as a heating roller, and a heater, such as halogen heater or an electromagnetic induction heating device, to heat the heating member. The image on the sheet is fixed with the heat from the heating member as well as the pressure from the pressure member where the pressure member presses against the heating member via the fixing member. Alternatively, the image may be fixed where the pressure member presses against a fixing belt that is heated by the heating roller.
It is known that the temperature of the heating member in the fixing device sharply rises after printing is completed. More specifically, during printing, that is, while the recording medium is being transported through the fixing device, because the recording medium draws heat from the heating member via the fixing member, the temperature of the heating member does not increase sharply.
By contrast, immediately after printing is completed, because the heat transfer to the recording medium suddenly stops, the temperature of the heating member can increase sharply due to, for example, the heat accumulated in a metal core of the heating member and so forth.
Because such an increase in the temperature of the heating member can be detected as an abnormal state, for example, in a known approach, it is avoided that such a temperature increase is detected as an abnormal state as follows: When the temperature of the heating member increases after printing is completed, the threshold value for detecting an sharply high temperature after a predetermined time has lapsed after the power-on is set to a relatively high temperature so that the temperature of a fixing roller after the power-on is not detected as abnormal.
However, if the temperature of the heating member increases significantly, it is not desirable because it can thermally damage the fixing device. This sharp increase in temperature is a phenomenon called “overshoot” of the heating member that occurs after printing is completed.
In particular, overshoot tends to occur immediately after completion of continuous printing. In addition, the higher the printing speed or CPM (Copies Per Minute) of the image forming apparatus, or the greater the weight per square of the recording medium, the more likely overshoot is to occur.
In view of the foregoing, various approaches described below have been advanced.
In known fixing devices, to prevent overshoot of the heating member after printing is completed, the temperature of the heating member is detected in a non-sheet area, where no recording medium contacts, after continuous printing is completed, and a cooling device to cool the heating member is operated when the detected temperature exceeds a predetermined temperature.
However, the known fixing devices described above have a drawback in that overshoot of the heating member can still occur in certain situations, such as when a main switch of the image forming apparatus is turned off during or immediately after printing is completed, when the conveyance of the recording medium is stopped (e.g., paper jamming occurs) during printing, or when the power supply to the image forming apparatus is stopped due to a power failure during or immediately after printing is completed.
If overshoot of the heating member occurs after or during printing, it is possible that a component constituting the fixing device may be thermally damaged. In particular, in an image forming apparatus whose printing speed is relatively high (e.g., approximately 75 CPM) and including a fixing device in which a fixing belt is stretched over a heating member such as a heating roller, the temperature of the fixing belt can rise to approximately 270 degrees Celsius due to overshoot of the heating member, at which temperature polyimide resin used for the fixing device deforms.
In view of the foregoing, there is a need to prevent overshoot of the heating member even when the main switch of the image forming apparatus is turned off or when power failure or paper jamming occurs, which the known development devices fail to do.